1. Field of the Invention
The present invention relates to a technique for controlling the gray level of an image that is displayed in response to an input signal.
2. Description of the Background Art
A display device for displaying images (including all visual information such as characters and symbols) is allowed to display an image with a gray level corresponding to the value of a signal inputted to the display device. The gray level relates to, for example, luminance or light transmittance. Here, the value of the signal inputted to the display device and the value of the gray level associate with a characteristic inherent to the display device (also referred to γ-characteristic: hereinafter, referred to as “display characteristic”). Therefore, in a display apparatus, provided with a display device, not a signal inputted to the display device (hereinafter, referred to as “input signal”) itself, but a conversion signal obtained by converting the input signal by using another characteristic (hereinafter, referred to as “correction characteristic”) is given, to the display device in order to allow the display device to display with a gray level that has a linear relationship with the value of the input signal.
Since there are variations in the display characteristic depending on display devices, the correction characteristic needs to be set for each of the display devices, and a look-up table (hereinafter, referred to as “LUT”) is used so as to make the input signal and the conversion signal associated with each other. Here, a storage device such as a memory in which the look-up table is stored is installed in the display apparatus as a conversion section together with the display device.
FIG. 8 is a graph that approximates the display characteristic. The value of a conversion signal is plotted on the axis of abscissa x and the value of a gray level is plotted on the axis of ordinate y. In order to approximate the display characteristic, the values of four conversion signals, x=P1, P2, R1, R2, are given to the display device, and the corresponding values of gray level y=Q1, Q2, S1, S2 are measured.
Here, in the range of x=0 to P2, the display characteristic is approximated by a function f(x), and in the range of x=P2 to R1, the display characteristic is approximated by a function g(x). In the range from x R1 to the maximum value of the conversion signal, the display characteristic is approximated by a function h(x). All of these functions f(x), g(x), and h(x) are, for example, functions containing logarithmic functions, and respectively determined by: (x, f(x))=(P1, Q1), (P2, Q2), (x, g(x))=(P2, Q2), (R1, S1), (x, h(x))=(R1, S1), (R2, S2).
FIG. 9 is a graph that shows a correction characteristic. The value of an input signal is plotted on the axis of abscissa x and the value of a conversion signal is plotted on the axis of ordinate y. In this case, the input signal and the conversion signal are both provided with 8 bit tones (28=256 gradient). Here, the input signal and the conversion signal are property conformed to each other in their ranges of variations, and with respect to correction characteristics, in the case of x=0 to Q2, f−1(x) is set, in the case of x=Q2 to S1, g−1(x) is set, and in the case of x=S1 to the maximum value of the input signal, h−1(x) is set, respectively. Here, the symbol “−1” represents the inverse function.
Therefore, by using an LUT having the correction characteristic shown in FIG. 9, the input signal is converted to the conversion signal, and based upon the resulting conversion signal, the display device executes a display with the gray level in accordance with the display characteristic; thus, the display is carried out with the gray level that has a linear relationship with the input signal. For example, such a technique has been disclosed by Japanese Patent Application Laid-Open No. 9-288468(1997).
However, in the technique proposed by Japanese Patent Application Laid-Open No. 9-288468(1997), the display characteristic is approximated by a function, and the inverse function to the function is calculated so as to provide a linear-type display characteristic. Then, this is multiplied by a coefficient for canceling a correction (hereinafter, referred to as “CRT-use y correction”) preliminarily applied to a video signal so as to correct the display characteristic of a cathode ray tube (CRT), if necessary, so as to set the LUT of the correction characteristic.
Therefore, only the LUT for setting the following relationships to a linear relationship has been provided: the relationship between the input signal and the gray level that is made associated with each other through the conversion signal or the relationship between the set value of the video signal prior to being applied the CRT-use γ correction and the gray level.